Highly crystalline thermoplastic copolyesters which crystallize rapidly and which are based on terephthalic acid, ethylene glycol and certain co-diols are known. DT-OS (German Published Specification) No. 2,507,674 relates to terephthalic acid copolyester consisting of at least 90 mol %, relative to the dicarboxylic acid component, of terephthalic acid radicals, 90 to 99.5 mol %, relative to the diol component, of ethylene glycol radicals and 0.5 to 10 mol %, relative to the diol component, of co-diol radicals, characterized in that (a) the co-diols employed carry two primary OH groups, (b) these two OH groups are separated by 3 or 4 C atoms and (c) the co-diols employed are either unsubstituted, monoalkyl-substituted or dialkyl-substituted, the alkyl groups carrying 2 or 3 C atoms, and, in the case of substitution, the sum of the C atoms of the alkyl substituents being at least 4.
DT-OS (German Published Specification) No. 2,507,776 relates to terephthalic acid copolyesters consisting of at least 90 mol %, relative to the dicarboxylic acid component, of terephthalic acid radicals, 90 to 99.5 mol %, relative to the diol component, of ethylene glycol radicals and 0.5 to 10 mol %, relative to the diol component, of co-diol radicals, characterized in that (a) the co-diols employed carry at least one secondary or tertiary OH group, (b) the co-diols employed contain 4 to 10 C atoms and (c) the two OH groups are separated by 3 to 4 C atoms.
These copolyesters can be molded at mold temperatures between 110.degree. and 150.degree. C., preferably at about 120.degree. C., and, under these conditions, permit a substantially shortened injection molding cycle time, compared with conventional polyethylene terephthalates.
The copolyesters according to DT-OS (German Published Specification) Nos. 2,507,674 and 2,507,776 can be prepared in a manner which is in itself known by esterifying or transesterifying the dicarboxylic acids, preferably pure terephthalic acid, and/or the corresponding dimethyl esters with 1.05 to 2.4, preferably 1.1 to 1.4, mols of the diols, relative to 1 mol of the dicarboxylic acid component, in the presence of esterification and/or transesterification catalysts between 150.degree. and 250.degree. C. (reaction step A) and subjecting the reaction products thus obtained to a polycondensation reaction under customary conditions, that is to say in the presence of esterification catalysts between 200.degree. and 300.degree. C. under reduced pressure (&gt;1 mm Hg) (reaction step B).
A particularly preferred embodiment consists in admixing the co-diols to the reaction mixture as late as possible, that is to say only after the reaction mixture of terephthalic acid, or of its ester-forming derivatives, with ethylene glycol to give bis-(2-hydroxyethyl) terephthalate has taken place, or, even more advantageously, only after a polyethylene terephthalate prepolymer which has a degree of polycondensation of more than 4 has formed. Thereafter, the mixture can be subjected to a polycondensation reaction in the customary manner, as described above.